1. Field of the Invention.
This invention relates in general to antenna systems, and in particular to an antenna element array alignment system.
2. Description of Related Art.
Communications satellites have become commonplace for use in many types of communications services, e.g., data transfer, voice communications, television spot beam coverage, and other data transfer applications. As such, satellites must provide signals to various geographic locations on the Earth""s surface. Typical satellites use customized antenna designs to provide signal coverage for a particular country or geographic area.
The primary design constraints for communications satellites are antenna beam coverage, isolation, and radiated Radio Frequency (RF) power. These two design constraints are typically thought of to be paramount in the satellite design because they determine which customers on the earth will be able to receive satellite communications service. Further, the satellite weight becomes a factor, because launch vehicles are limited as to how much weight can be placed into orbit.
Many satellites operate over fixed coverage regions and employ polarization techniques, e.g., horizontal and vertical polarized signals, or circularly polarized signals, to increase the number of signals that the satellite can transmit and receive. These polarization techniques use a single unshaped parabolic mesh reflector with offset focus points to produce substantially congruent coverage regions for the polarized signals. This approach is limited because the coverage regions are fixed and cannot be changed on-orbit, and the cross-polarization isolation for wider coverage regions is limited to the point that many satellite signal transmission requirements cannot increase their coverage regions.
Many satellite systems would be more efficient if they contained antennas with high directivity of the antenna beam and had the ability to have the coverage region be electronically configured on-orbit to different desired beam patterns. These objectives are typically met using a phased array antenna system. However, phased array antennas carry with them the problems of large signal losses between the power amplifiers and the antenna horns, and difficult integration and test measurements and characterization.
During the design and test of a phased arrays system, the phased array antenna system is mated with power amplifiers, typically Solid-State Power Amplifiers (SSPAs) to determine the RF power output of the system. Although the power is directly measured during SSPA output, the SSPA is in the compression (saturation) region during this measurement. It is preferable to measure the SSPA in the linear region. The SSPA is better measured in the linear region, when there are no signals travelling through the SSPA, but this is not practical to do during testing of the spacecraft. If the SSPA is properly characterized, the Signal-to-Noise Ratio (SNR) can be improved through continuous time integration of the signal.
It can be seen, then, that there is a need in the art for antenna systems that can measure the SSPA while communications signals are travelling through the system. It can also be seen that there is a need in the art for antenna systems that are characterized properly to improve the SNR of the communications signals.
To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses methods and an apparatus for characterizing an antenna system. The apparatus comprises a processor, a coupler, and a converter. The processor selectively injects a test signal into amplifiers in the antenna system while other amplifiers are amplifying the broadcast signal, and the amplified signals are then fed to a hybrid matrix. The coupler samples the combined amplified test and broadcast signals, and the converter converts the combined test and broadcast signals to a different frequency band to separate the test signal from the broadcast signal. The processor determines a phase response of the first amplifier and a phase effect of the hybrid matrix by measuring the separated test signal and modifies a phase of the broadcast signal using the determined phase response of the first amplifier and the hybrid matrix when the broadcast signal is subsequently provided to the first amplifier.
The method comprises the steps of preventing a first amplifier from receiving a broadcast signal, injecting a test signal into the first amplifier, amplifying the broadcast signal by at least a second amplifier, combining the amplified test signal with the amplified broadcast signal, monitoring the combined amplified test signal, separating the combined amplified test signal to retrieve the amplified test signal, measuring the separated amplified test signal to determine a phase response and an amplitude of the first amplifier and a phase effect of the combining step, and modifying a phase of the broadcast signal using the determined phase response and the phase effect when the broadcast signal is subsequently provided to the first amplifier.
The present invention provides antenna systems that can measure the SSPA while communications signals are travelling through the system. The present invention also provides antenna systems that are characterized properly to improve the SNR of the communications signals.